Credits are the single unit for everything the studio generates. Each action spends a number of them, and that number tracks what the underlying model actually costs to run — so a short, fast video costs less than a long one at high resolution, and a premium image model costs more than a lightweight one.
This is why the cost is shown per action rather than hidden behind an unlimited-sounding plan. Video is the clearest example: duration, resolution and whether you generate audio each move the price, because each of them moves the provider's cost.
New accounts start with free credits, and every plan tops up a monthly balance. Practically, the way to spend fewer credits is to iterate on cheaper models and switch to an expensive one for the final render — not to ration your experiments.